1. Field of the Invention
The present invention relates to an image reading apparatus and a controlling method thereof.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2007-097054 discloses a image reading apparatus that includes a one-line contact image sensor (CIS) and a plurality of light sources, each of which emits one of red light, green light, and blue light. The one-line CIS includes a plurality of image sensing elements such as charged coupled devices (CCDs) that are linearly arranged to form a one-line array. A print medium is read by the image reading apparatus by sequentially turning on the light sources and exposing the image sensing elements to light. Upon being exposed to light, each image sensing element generates R line data corresponding to red light, G line data corresponding to green light, and B line data corresponding to blue light (hereinafter, “RGB line data”). The one-line CIS outputs the RGB line data to a control unit. The control unit first generates RGB pixel data corresponding to each image sensing element based on the RGB line data and then generates RGB read-image data based on the RGB pixel data.
The print medium is carried with respect to the one-line CIS such that the one-line CIS can read the print medium. That is, the print medium is in a moving state while the one-line CIS reads the print medium. Thus, the portion on the print medium facing the one-line CIS during the ON period of each light source is different. That results in a color mismatch between the color of the RGB pixel data and the color of the corresponding portion on the print medium. Hence, color shift of characters or color moire occurs in the RGB read-image data. Particularly in the case of low image reading resolution, the print medium moves faster with respect to the one-line CIS, i.e., the portion on the print medium facing the one-line CIS becomes largely-spaced thereby making the color shift more prominent.
Such color shift in the RGB read-image data can be suppressed by linearly arranging the image sensing elements to form a three-line array. However, a three-line array arrangement of the image sensing elements is expensive as compared to a one-line array arrangement. That results in high manufacturing cost for the image reading apparatus.
The color shift can also be suppressed by configuring the image reading apparatus to read the print medium at a reading resolution higher than a specified reading resolution, generate RGB read-image data corresponding to the higher reading resolution, and then correct the generated RGB read-image data to RGB read-image data corresponding to the specified reading resolution. However, while reading the image print at a higher reading resolution, the carrying speed of the print medium with respect to the one-line CIS slows down than in the case of reading the image print at the specified reading resolution.